There is different types of causes of data loss, but the most common ones are Hardware or System Malfunctions and Human Error.
The hardware or system malfunctions can be shown in many forms for example, electrical failure, head crash controller failure etc.
What I mean about human errors is that people sometimes accidentally clicking the delete or format button to erase something we didn't meant to or another one by is causing physical damages of dropping or failing our storage by accident
Hope this helped ^.^
